Carbon fiber-reinforced plastics using carbon fibers as reinforcing fibers (hereinafter called CFRPs) are excellent in specific strength and specific modulus, and hence are used as aircraft members, since the fuel consumption can be remarkably improved because of their lightweight. Especially as the secondary structural materials such as the flaps, fairings and ailerons and interior materials such as toilet rooms, ceiling panels and luggage compartments respectively of aircraft, used are honeycomb sandwich structural bodies, each comprising a core member having a honeycomb structure and plate-like skin members bonded to both the surfaces of the core member. CFRPs are popularly used as members constituting the skin members.
A honeycomb sandwich structural body is formed by overlaying a prepreg composed of a carbon fiber woven fabric as skin member on a core member having a honeycomb structure, and curing a thermosetting resin in the prepreg simultaneously with bonding between the honeycomb structure and CFRP under heating and pressurizing. This production method is usually called the honeycomb co-curing method.
The conventional carbon fiber woven fabric used as a component of a prepreg in the honeycomb co-curing method is as shown in FIG. 5. A carbon fiber woven fabric 50 shown in FIG. 5 comprises carbon fiber warp yarns 51 and carbon fiber weft yarns 52, and has large open spaces (gaps) 56 formed near the intersections between the warp yarns 51 and the weft yarns 52, where no carbon fibers exist. The prepreg is formed by impregnation of a matrix resin into the carbon fiber woven fabric 50.
When the prepreg is molded to produce a honeycomb sandwich structural body, the prepreg is pressurized in the portions in contact with the honeycomb core, but is not pressurized at the portions not in contact with the core. In this production process, the gaps 56 of the carbon fiber woven fabric 50 are likely to be devoid of the resin. If the voids are devoid of the resin, there arises a problem that the skin members of the produced honeycomb sandwich structural body has holes formed. If such a defective honeycomb sandwich structural body is used, water goes from the holes into the cavities of the honeycomb, and there arises a serious problem that if the water is frozen to expand, the honeycomb sandwich structural body may be destroyed.
To avoid this problem, it is practiced to use a woven fabric composed of relative thin carbon fiber yarns, each consisting of 3,000 filaments and having a relatively small fineness of 1,800 deniers, as the woven fabric for the honeycomb co-curing method. The carbon fiber woven fabric is small in the size of open spaces.
However, the productivity of such thin carbon fiber yarns is low, and the speed at which a woven fabric composed of them is produced is also low. So, the honeycomb sandwich structural is body produced using the woven fabric has a problem of high cost, even though it is immune from the aforesaid problem.
On the other hand, to raise the productivity and to obtain low cost CFRP, it is advantageous to produce thick carbon fiber yarns and to use them for producing a woven fabric. However, in the case where a thin woven fabric having a low unit weight is produced using thick carbon fiber yarns, there arises a problem that the produced woven fabric has large gaps formed near the interlaced portions between warp yarns and weft yarns, where no carbon fibers exist.
Furthermore, JP 7-300739 A proposes a woven fabric composed of thick carbon fiber yarns in each of which carbon fibers forming the yarn are uniformly and flatly dispersed. In the woven fabric, since the warp yarns and the weft yarns are interlaced with each other in such a state that the carbon fiber yarns used as the warp yarns and the weft yarns are kept flat in cross sectional form, the crimps formed by the warp yarns and the weft yarns are small, and the mechanical properties of the CFRP produced using the woven fabric are very excellent.
However, since the warp yarns and the weft yarns of the flat carbon fiber yarn woven fabric are interlaced with each other with crimps slightly formed, the forces for placing positions of the warp yarns or the weft yarns under restraint at the interlaced portions are extremely small. In the case where this woven fabric is processed by a wet-prepreg processing, when the solvent is dried away, the flat warp yarns and the flat weft yarns are deformed and bundled roundly due to the surface tension of the resin. As a result, a prepreg having large open spaces is produced as a problem of this technique.
To overcome the problem, JP 10-317250 A proposes a woven fabric, in which auxiliary yarns obtained by covering hardly heat shrinkable yarns with an adhesive polymer are paralleled with carbon fiber warp yarns or carbon fiber weft yarns, to bond the warp yarns and the weft yarns to each other at their intersections by the adhesive polymer. If the warp yarns and the weft yarns are bonded to each other at their intersections like this, the degree of yarn bundling in the yarn width direction caused when the solvent is dried away in a wet-prepreg processing process can be lowered.
However, since the auxiliary yarns covered with an adhesive polymer are paralleled with the carbon fiber yarns, they can migrate into the carbon fiber yarns, and all the intersections between the warp yarns and the weft yarns cannot be stably bonded. So, there arises a problem that large open spaces can be partially formed in a wet-prepreg processing. Furthermore, the polymer used as an adhesive exists almost in the central portions in the yarn width direction of the warp yarns and the weft yarns, but does not exist near the gaps 56 formed due to the interlacing between the warp yarns and the weft yarns. So, there arises a problem that a wet-prepreg processing is likely to expand the gaps 56 small in the beginning.
As described above, even though a flat carbon fiber yarn woven fabric is excellent in performance and cost, there has been no method of reliably preventing the formation of open spaces in a wet-prepreg processing. That is, the above-mentioned prior art does not solve the problem that open spaces are formed in a wet-prepreg.
It would accordingly be helpful to provide a carbon fiber-made reinforcing woven fabric comprising warp yarns, each of which is composed of carbon fibers, and weft yarns, each of which is composed of carbon fibers, wherein dispersion of the carbon fibers is substantially uniform in the plane direction of the carbon fiber woven fabric, and wherein gaps formed near the interlaced portions between the warp yarns and the weft yarns are small and a cover factor is large.
It would also be helpful to provide a prepreg produced by processing the carbon fiber-made reinforcing woven fabric of the invention under a wet-prepreg processing, and also to provide a method for producing the prepreg.